Novel Highly Selective and Reversible Chemosensors Based on Dual-Ratiometric Fluorescent Electrospun Nanofibers with pH- and Fe<sup>3+</sup>-Modulated Multicolor Fluorescence Emission

Abstract

Novel dual-ratiometric fluorescent electrospun (ES) nanofibers featuring high sensitivity for pH and ferric ion (Fe<sup>3+</sup>) were prepared using binary blends of poly­(2-hydroxyethyl methacrylate<i>-<i>co</i>-N</i>-methylolacrylamide<i>-<i>co</i>-</i>nitrobenzoxadiazolyl derivative) (poly­(HEMA<i>-<i>co</i>-</i>NMA<i>-<i>co</i>-</i>NBD)) and a spirolactam rhodamine derivative (SRhBOH) by employing a single-capillary spinneret. The HEMA, NMA, and NBD moieties were designed to exhibit hydrophilic properties, chemical cross-linking, and fluorescence (fluorescence resonance energy transfer (FRET) donor), respectively. The fluorescence emission of SRhBOH was highly selective for pH and Fe<sup>3+</sup>; when SRhBOH detected acidic media and Fe<sup>3+</sup>, the spirocyclic form of SRhBOH, which is nonfluorescent, was transformed into the opened cyclic form and exhibited strong fluorescence emission. The emission colors of ES nanofibers in acidic or Fe<sup>3+</sup> aqueous solutions changed from green to red because of FRET from NBD (donor) to SRhBOH (acceptor). The off/on switching of the FRET process was modulated by adjusting the SRhBOH blending ratio, pH, and Fe<sup>3+</sup> concentration. Poly­(HEMA<i>-<i>co</i>-</i>NMA<i>-<i>co</i>-</i>NBD) ES fibers blended with 20% SRhBOH showed high sensitivity in sensing Fe<sup>3+</sup> and pH because of the substantial 57 nm red shift in emission as well as substantial reversible dual photoluminescence. The prepared FRET-based dual-ratiometric fluorescent ES nanofibrous membranes can be used as “naked eye” sensors and have potential for application in multifunctional environment sensing devices

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